Curtain coating process

ABSTRACT

Webs coated with several layers, of which the lowermost or uppermost layer contains an instant hardener, may be produced in a single operation by curtain coating using a V-coater when the hardening layer is guided at the negatively inclined sliding surface of the V-coater while the other layers are guided at the opposite sliding surface of the V-coater and the hardening layer is combined with the remaining layers at coating edge to form a layer packet which is applied as a free-falling curtain to the web to be coated.

This invention relates to a process for the production of webs coatedwith several layers, the lowermost or uppermost layer being a layercontaining an instant hardener (hardening layer).

For the production of photographic materials, the layers have to behardened, for which purpose the layers contain protein- and/orpolymer-based binders. The following processes are known for hardening:

Conventional hardening with additives, for example triacrylformal, whichare added to the coating solutions before they are applied to the web.The web has to be stored for several weeks before hardening of theapplied layer packet is complete. The disadvantage of this process liesin the high storage costs and the variable quality attributable to thevariable hardening caused by different storage conditions.

The rapid hardening process also uses additives, for example certainvinylsulfones, which are added to the coating solutions. The hardeningof the layer packet is complete at the earliest after one day and at thelatest after eight days, which is basically of advantage, although verystringent demands are imposed upon the drying and residual moisturecontent of the products because otherwise the quality of the products isadversely affected by posthardening.

The instant hardening process differs fundamentally from the other twoprocesses. Instant hardeners are understood to be compounds whichcrosslink suitable binders in such a way that, immediately after coatingor after 24 hours at the latest and preferably after 8 hours, hardeninghas advanced to such an extent that no further change in thesensitometry or swelling of the layer packet is caused by thecrosslinking reaction. By swelling is meant the difference between wetlayer thickness and dry layer thickness during the aqueous processing ofthe film (Photogr. Sci. Eng. 8 (1964), 275; Photogr. Sci. Eng. 16(1972), 449).

Because hardening begins immediately, the instant hardener cannot beadded to the usual gelatin-containing coating solutions, becauseotherwise the production process would have to be interrupted after ashort time for cleaning to remove hardener material from the coatingmachine.

Accordingly, in the instant hardening process, which is of considerableadvantage in terms of product quality, the hardener has to be applied ina separate coating solution containing from 0 to 4% by weight andpreferably from 0.5 to 1% by weight gelatin or gelatin derivativeseither as an additional layer or as part of a layer packet, in which theadjacent layers also have gelatin concentrations of at most only 4% byweight. Where gelatin derivatives are used, they may be present inhigher concentrations in the adjacent layers.

Without the above-mentioned restrictions in regard to the gelatinconcentration, the only possible solutions are technically moreinvolved. For example, the hardener solution may be applied at a secondcoating station after drying of the first coating or by separate passagethrough the machine.

All the disadvantages mentioned above are obviated by the inventiondescribed hereinafter.

Accordingly the object of the present invention is to improve theinstant hardening process in such a way that the hardener solution maybe applied together with all other layers and, hence, with thegelatin-rich coating packets as well in a single coating operationcarried out in a single coating machine.

It has surprisingly been found that the instant hardener solution may beapplied together with the layer packet using the curtain coaterdescribed in DE-OS No. 3 238 905 C2 without any local incrustations,providing the hardener solution is delivered through the rear outletslot 9.1 described therein and only contacts the layer packet in thecurtain. Surprisingly, no coating problems or losses of quality occur atthe coating edge where the hardener solution and layer packet cometogether.

Accordingly, the present invention relates to a process for theproduction of a web coated with at least two layers, at least one layercontaining protein- and/or polymer-based binders and another layer,applied as lowermost or uppermost layer, containing an instant hardener(hardening layer) by the curtain coating process using a V-coater pastwhich the web is continuously guided, characterized in that thehardening layer applied at the same time as the binder-containing layeris guided at the negatively inclined sliding surface of the V-coaterwhile the binder-containing layer is guided at the opposite slidingsurface of the V-coater and the hardening layer is combined with thebinder-containing layer at the coating edge to form a layer packet whichis applied as a free-falling curtain to the web to be coated.

The V-coater is preferably combined with a curtain holder withintegrated liquid supply of the type described in U.S. Pat. No.4,479,987. This curtain holder has the additional advantage that, byvirtue of the continuous rinsing, no hardening or incrustation caused bythe hardener can form thereon.

In addition, the measure according to the invention of applying theinstant hardener solution and the layer packet in a single coatingprocess eliminates the need for the acceleration or wetting layerdescribed in DE No. 3 238 905 C2, because the hardener solution alsoperforms that function This is possible by guiding the web in such a waythat the hardener solution is situated beneath the layer packet ascoupling layer between the layer packet and the surface of the web (seeFIG. 1, solid-line direction of travel of the web). Alternatively, itmay form the uppermost layer in the opposite direction of travel of theweb (see FIG. 1, chain-line direction of travel of the web) and, hence,may be used as the top layer with surface-specific additives.

Considerable and surprising advantages of the process according to theinvention include the saving of a second coating station or of a secondpassage through the machine for the application of the hardenersolution; the avoidance of adjacent layers poor in gelatin and thesaving of expensive hardener because the quantity of hardener can besubstantially halved without any loss of hardening effect.

FIG. 1 is a section through a curtain coating machine of the V-coatertype. This coater consists of the blocks 13 and 14 which are screwed toone another and bounded by end plates. The end plates and also the meansby which the coater is fastened to a frame have not been shown. Thehardener solution 7 and the liquid coating solutions 8 for the layerpacket 11 are introduced into the distributing chamber 5 at one end bymeans of known dosing units and pipes which it is not intended todescribe here. In conjunction with the distributing chambers 5, theoutlet slots 9.1 to 9.5 provide for uniform distribution of the coatingsolutions 8 and of the hardener solution 7. The coating solutions 8 forthe layer packet issue from the outlet slots 9.2-9.5 and flow by gravitydown the cascade surfaces 3 at an angle α₁. Finally, the layer packet 11flows over the surface 15 to the lowermost V-shaped coating block 14 andto the coating edge 4.

The hardener solution 7 is fed to the distributing chamber 5 between thecoating blocks 13 and 14 and flows out through the outlet slot 9.1 ontoa sliding surface 16 negatively inclined at an angle α₂. It follows thesliding surface 16 and flows from the other side of the coating block 14to the common coating edge 4. At the coating edge 4, the free-fallingcurtain 12 is formed from the first-mentioned layer packet 11 and thehardening layer 7. The free-falling curtain 12 reaches the web 1 to becoated in fractions of a second over the height h and applies itself tothe web 1. The moving web 1 is supported by the coating rollers 6 wherethe curtain 12 impinges on it while the edges are held in known mannerby curtain guides (not shown). To avoid local hardening, an aqueousinert layer may be arranged between the curtain guides and the layers.

The curtain 12 coats the web 1 over its entire width. Surplus coatingmaterial may be drained off at the edges through collecting troughs.Webs without any cut edges are thus formed, being coated with thecoating solutions over their entire width without any edge loss.

However, the web 1 is advantageously coated to just short of its edges,for which purpose the curtain 12 is guided in known manner by curtainguide elements extending almost to the web and is thus prevented fromcontracting through surface tension. Less of the valuable coatingsolution is lost in this way. The coated web 1 with the coating 2 isthus not coated over its entire width and has to be cut, the uncoatededges and the edge beads being cut off.

FIG. 1 shows chain-line and solid-line directions of travel of the webwhich are to be interpreted as alternatives. If required or ifnecessary, the hardener layer 7 may be placed beneath the layer packet11 or on the layer packet 11.

Although the instant hardener contacts the layer packet 11 in thecurtain 12 and at the coating edge 4, no incrustation builds up so thatthe quality of coating is not affected. The high coating speedsmentioned in DE No. 3 238 905 C2 are maintained.

The hardening layer preferably has a viscosity of from 1 to 30 mPa.s anda wet layer thickness of from 3 to 30 μm; the other layers preferablyhave viscosities of from 10 to 500 mPa.s and web layer thicknesses offrom 5 to 100 μm.

At least one of the other layers preferably contains gelatin and aphotosensitive silver halide, while the hardening layer contains from 0to 4% by weight and preferably from 0.5 to 1% by weight gelatin.

The coating edge is situated in particular 10 to 100 mm above thesurface of the web to be coated.

Suitable examples of instant hardeners are compounds corresponding tothe following general formulae: ##STR1## in which

R₁ represents alkyl, aryl or aralkyl,

R₂ has the same meaning as R₁ or represents alkylene, arylene,aralkylene or alkaralkylene, the second bond being attached to the groupcorresponding to the following formula ##STR2## or

R₁ and R₂ together represent the atoms required to complete anoptionally substituted heterocyclic ring, for example a piperidine,piperazine or morpholine ring, the ring optionally being substituted,for example, by C₁ -C₃ alkyl or halogen,

R₃ represents hydrogen, alkyl, aryl, alkoxy, --NR₄ --COR₅, --(CH₂)_(m)--NR₈ R₉, --(CH₂)_(n) --CONR₁₃ R₁₄ or ##STR3## or a bridge member or adirect bond to a polymer chain,

R₄, R₆, R₇, R₉, R₁₄, R₁₅, R₁₇, R₁₈ and R₁₉ being hydrogen or C₁ -C₄alkyl,

R₅ represents hydrogen, C₁ -C₄ alkyl or NR₆ R₇,

R₈ -COR₁₀,

R₁₀ represents NR₁₁ R₁₂,

R₁₁ represents C₁ -C₄ alkyl or aryl, more especially phenyl,

R₁₂ represents hydrogen, C₁ -C₄ alkyl or aryl, more especially phenyl,

R₁₃ represents hydrogen, C₁ -C₄ alkyl or aryl, more especially phenyl,

R₁₆ represents hydrogen, C₁ -C₄ alkyl, COR₁₈ or CONHR₁₉,

m is a number of from 1 to 3,

n is a number of from 0 to 3,

p is a number of from 2 to 3 and

Y represents 0 or NR₁₇ or

R₁₃ and R₁₄ together represent the atoms required to complete anoptionally substituted heterocyclic ring, for example a piperidine,piperazine or morpholine ring, the ring optionally being substituted,for example, by C₁ -C₃ alkyl or halogen,

Z represents the carbon atoms required to complete a 5-membered or6-membered aromatic heterocyclic ring, optionally with a linked benzenering, and

X.sup.⊖ is an anion which is dropped if an anionic group is alreadyattached to the remainder of the molecule; ##STR4## in which

R₁, R₂, R₃ and X.sup.⊖ have the same meanings as defined for formula(a); ##STR5## in which

R₂₀, R₂₁, R₂₂, R₂₃ represent C₁ -C₂₀ alkyl, C₆ -C₂₀ aralkyl, C₅ -C₂₀aryl, either unsubstituted or substituted by halogen, sulfo, C₁ -C₂₀alkoxy, N,N-di-C₁ -C₄ -alkyl-substituted carbamoyl and, in the case ofaralkyl and aryl, by C₁ -C₂₀ alkyl,

R₂₄ is a group releasable by a nucleophilic agent and

X.sup.⊖ has the same meaning as defined for formula (a); 2 or 4 of thesubstitutents R₂₀, R₂₁, R₂₂ and R₂₃ may even be combined together with anitrogen atom or the group ##STR6## optionally with inclusion of furtherheteroatoms, such as O or N, to form one or two saturated 5-7-memberedrings;

    R.sub.25 --N═C═N--R.sub.26                         (d)

in which

R₂₅ represents C₁ -C₁₀ alkyl, C₅ -C₈ cycloalkyl, C₃ -C₁₀ alkoxyalkyl orC₇ -C₁₅ aralkyl,

R₂₆ has the same meaning as R₂₅ or represents a group corresponding tothe following formula ##STR7## where

R₂₇ is C₂ -C₄ alkylene and

R₂₈, R₂₉ and R₃₀ represent C₁ -C₆ alkyl; one of the groups R₂₈, R₂₉ andR₃₀ may be substituted by a carbamoyl group or a sulfo group and two ofthe groups R₂₈, R₂₉ and R₃₀ may even be attached, together with thenitrogen atom, to form an optionally substituted heterocyclic ring, forexample a pyrrolidine, piperazine or morpholine ring which may besubstituted, for example, by C₁ -C₃ alkyl or halogen, and

X.sup.⊖ has the same meaning as defined for formula (a); ##STR8## inwhich

X.sup.⊖ has the same meaning as defined for formula (a),

R₂₄ has the same meaning as defined for formula (c),

R₃₁ represents C₁ -C₁₀ alkyl, C₆ -C₁₅ aryl or C₇ -C₁₅ aralkyl, eitherunsubstituted or substituted by carbamoyl, sulfamoyl or sulfo,

R₃₂ and R₃₃ represent hydrogen, halogen, acylamino, nitro, carbamoyl,ureido, alkoxy, alkyl, alkenyl, aryl or aralkyl or together form theremaining members of a ring, more especially a benzene ring, fused tothe pyridinium ring;

R₂₄ and R₃₁ may be attached to one another where R₂₄ is a sulfonyloxygroup; ##STR9## in which

R₁, R₂ and X.sup.⊖ have the same meaning as defined for formula (a) and

R₃₄ represents C₁ -C₁₀ alkyl, C₆ -C₁₄ aryl or C₇ -C₁₅ aralkyl; ##STR10##in which

R₁, R₂ and X.sup.⊖ have the same meaning as defined for formula (a),

R₃₅ represents hydrogen, alkyl, aralkyl, aryl, alkenyl, R₃₈ O--, R₃₉ R₄₀N, R₄₁ R₄₂ C═N-- or R₃₈ S--,

R₃₆ and R₃₇ represent alkyl, aralkyl, aryl, alkenyl, ##STR11## R₄₄ --SO₂or

R₄₅ --N═N-- or, together with the nitrogen atom, represent the remainingmembers of a heterocyclic ring or the group ##STR12##

R₃₈, R₃₉, R₄₀, R₄₁, R₄₂, R₄₃, R₄₄ and R₄₅ represent alkyl, aralkyl,alkenyl, in addition to which R₄₁ and R₄₂ may represent hydrogen; inaddition, R₃₉ and R₄₀ or R₄₁ and R₄₂ may represent the remaining membersof a 5- or 6-membered, saturated carbocyclic or heterocyclic ring;##STR13## in which

R₄₆ represents hydrogen, alkyl or aryl,

R₄₇ represents acyl, carbalkoxy, carbamoyl or aryloxycarbonyl;

R₄₈ represents hydrogen or R₄₇

R₄₉ and R₅₀ represent alkyl, aryl, aralkyl or, together with thenitrogen atom, represent the remaining members of an optionallysubstituted heterocyclic ring, for example a piperidine, piperazine ormorpholine ring, which may be substituted for example by C₁ -C₃ alkyl orhalogen, and

X.sup.⊖ has the same meaning as defined for formula (a);

    R.sub.51 --SO.sub.2 --CH═CH.sub.2 ].sub.q              (i)

in which

R₅₁ is an optionally substituted heteroaromatic ring containing at leastq ring-C-atoms and at least one ring-O-, ring-S- or ring-N-atom and

q is an integer of ≧2.

The heteroaromatic ring represented b R₅₁ is, for example, a triazole,thiadiazole, oxadiazole, pyridine, pyrrole, quinoxaline, thiophene,furan, pyrimidine or triazine ring. In addition to the at least twovinylsulfonyl groups, it may optionally contain further substituentsand, optionally, fused benzene rings which, in turn, may also besubstituted. Examples of heteroaromatic rings (R₅₁) are shown in thefollowing: ##STR14## in which

r is a number of from 0 to 3 and

R₅₂ represents C₁ -C₄ alkyl, C₁ -C₄ alkoxy or phenyl.

Finally, suitable instant hardeners are the compounds described in JP-OSNos. 38 540/75, 93 470/77, 43 353/81 and 113 929/83 and in U.S. Pat. No.3,321,313.

Unless otherwise defined, alkyl is, in particular, C₁ -C₂₀ alkyloptionally substituted by halogen, hydroxy, sulfo, C₁ -C₂₀ alkoxy.

Unless otherwise defined, aryl is, in particular, C₆ -C₁₄ aryloptionally substituted by halogen, sulfo, C₁ -C₂₀ alkoxy or C₁ -C₂₀alkyl. Unless otherwise defined, aralkyl is, in particular, C₇ -C₂₀aralkyl optionally substituted by halogen, C₁ -C₂₀ alkoxy, sulfo or C₁-C₂₀ alkyl. Unless otherwise defined, alkoxy is in particular C₁ -C₂₀alkoxy.

X.sup.⊖ is preferably a halide ion, such as Cl.sup.⊖, Br.sup.⊖ orBF₄.sup.⊖, NO₃.sup.⊖, (SO₄ ²⊖)_(1/2), ClO₄.sup.⊖, CH₃ OSO₃.sup.⊖,PF₆.sup.⊖, CF₃ SO₃.sup.⊖.

Alkenyl is in particular C₂ -C₂₀ alkenyl. Alkylene is in particular C₂-C₂₀ alkylene. Arylene is in particular phenylene. Aralkylene is inparticular benzylene and alkaralkylene is in particular xylylene.

Suitable N-containing ring systems which may stand for Z are shown onpages 11 and 12. The pyridine ring is preferred.

R₃₆ and R₃₇ together with the nitrogen atom to which they are attachedform in particular a pyrrolidine or piperidine ring having two oxogroups attached in the o- and o'- positions, which ring may be benzo-,cyclohexano- or [2.2.1]-bicyclohexene-condensed.

Acyl is in particular C₁ -C₁₀ alkylcarbonyl or benzoyl: carbalkoxy is inparticular C₁ -C₁₀ alkoxycarbonyl; carbamoyl is in particular mono- ordi-C₁ -C₄ -alkylaminocarbonyl; carboxyl is in particularphenoxycarbonyl.

Groups R₂₄ releasable by nucleophilic agents are, for example, halogenatoms, C₁ -C₁₅ alkylsulfonyloxy groups, C₇ -C₁₅ aralkylsulfonyloxygroups, C₆ -C₁₅ arylsulfonyloxy groups and 1-pyridinyl radicals.

Preferred hardeners are listed in the following:

Compounds according to formula (a) ##STR15##

Syrup, highly hygroscopic ##STR16##

Syrup, highly hygroscopic ##STR17##

The compounds may be readily obtained by methods known from theliterature. The carbamic acid chlorides are prepared from the secondaryamines, for example with phosgene, and are then reacted with aromatic,heterocyclic nitrogen-containing compounds in the absence of light. Thepreparation of compound 3 is described in Chemische Berichte 40, (1907),page 1831. Further information on the synthesis can be found in DE-OSNo. 2 225 230, DE-OS No. 2 317 677 and DE-OS No. 2 439 551.

Compounds corresponding to formula (b)

Processes for the synthesis of these compounds are described, forexample, in De-A No. 2 408 814: ##STR18##

Compounds corresponding to formula (c)

Methods for the synthesis of these compounds are described in detail inChemistry Letters (The Chemical Society of Japan), page 1891-1894(1982). Further particulars of the synthesis can also be found in EP-ANo. 162 308. ##STR19##

Compounds corresponding to formula (d)

Methods for the synthesis of these compounds are described in detail inJP-OS Nos. 126 125/76 and 48 311/77. ##STR20##

Compounds corresponding to formula (e)

Methods for the synthesis of these compounds are described in detail inJP-OS Nos. 44 140/82 and 46 538/82 and in JP-OS No. 50 669/83. ##STR21##

Compounds corresponding to formula (f)

Methods for the synthesis of these compounds are described in detail inJP-OS No. 54 427/77. ##STR22##

Compounds corresponding to formula (g)

The synthesis of these compounds is described in U.S. Pat. No.4,612,280. ##STR23##

Compounds corresponding to formula (h)

The preparation of these compounds is described in DD No. 232 564 A 1.##STR24##

Compounds corresponding to formula (i)

Methods for the preparation of these compounds are described in DE-OSNo. 35 23 360. ##STR25##

Other suitable instant hardeners correspond to the following formulae:##STR26##

The symbols used in the following Table have the following meanings:

    ______________________________________                                        η = viscosity       (mPa.s)                                               σ = surface tension                                                                             (mN/m)                                                δ = wet coating on web                                                                          (μm)                                               ν = speed of travel of web                                                                         (m/min.)                                              h = curtain height      (mm)                                                  ______________________________________                                    

A coating machine of the type shown in FIG. 1 (V-coater) was used for aneight-layer coating (one instant hardened layer, 7-layer photographiclayer packet).

The coating data of the individual layers were as follows:

    ______________________________________                                        Outlet                                                                        slot  Nature of coating solution                                                                     δ                                                                              η                                                                             σ                                                                            h   ν                               ______________________________________                                        9.1   Instant hardener solution                                                                       7      3  30                                          9.2   Photographic layer                                                                             15     40  35                                          9.3   Intermediate layer                                                                             15     40  35                                          9.4   Photographic layer                                                                             15     40  35   70  200                                9.5   Intermediate layer                                                                             15     40  35                                          9.6   Photographic layer                                                                             15     40  35                                          9.7   Intermediate layer                                                                             15     30  35                                          9.8   Cover layer      10     10  32                                          ______________________________________                                    

The direction of travel of the web was selected so that the instanthardener layer was applied as the uppermost layer of the coating 2 (FIG.1, direction of web travel in chain lines). A PE-paper substrate wasused as the web.

The coating quality was good and no incrustations were formed on thecoater. An aqueous solution of the following compound ##STR27## was usedas the instant hardener solution.

Where 2 coating stations or two machine passes were used, the samecoating quality was only achieved with a 12 μm application of hardener,7 μm in accordance with the invention.

According to the invention, less or more than 8 layers may be applied.The process is particularly suitable for applying 2 to 20 layers.

We claim:
 1. A process for the production of a web coated with at leasttwo layers, at least one layer containing binders based on proteinsand/or synthetic polymers and another layer, applied as the lowermost oruppermost layer, containing an instant hardener (hardening layer), bythe curtain coating method using a V-coater past which the web iscontinuously guided, characterized in that the hardening layer appliedat the same time as the binder-containing layer is guided at thenegatively inclined sliding surface of the V-coater while thebinder-containing layer is guided at the opposite sliding surface of theV-coater and the hardening layer is combined with the binder-containinglayer at the coating edge to form a layer packet which is applied as afree-falling curtain to the web to be coated.
 2. A process as claimed inclaim 1, characterized in that the layer containing the instant hardenerhas a viscosity of from 1 to 30 mPa.s and a wet layer thickness of from3 to 30 μm.
 3. A process as claimed in claim 1, characterized in that atleast one of the other layers contains gelatin and a photosensitivesilver halide.
 4. A process as claimed in claim 1, characterized in thatthe instant hardener is applied in a coating solution containing from 0to 4% by weight gelatin.
 5. A process as claimed in claim 1,characterized in that the instant hardener is applied in a coatingsolution containing a gelatin derivative.
 6. A process as claimed inclaim 1, characterized in that the coating edge is situated 10 to 100 mmabove the surface of the web to be coated.
 7. A process as claimed inclaim 1, characterized in that the curtain is held by a curtain holderwith integrated liquid supply.
 8. A process as claimed in claim 1,characterized in that the web is coated with 2 to 20 layers.
 9. Aprocess as claimed in claim 1, characterized in that the instanthardener corresponds to the following general formula ##STR28## in whichR₁ represents alkyl, aryl or aralkyl,R₂ has the same meaning as R₁ orrepresents alkylene, arylene, aralkylene or alkaralkylene, the secondbond being attached to a group of the formula ##STR29## or R₁ and R₂together represents the atoms required to complete an optionallysubstituted heterocyclic ring, R₃ represents hydrogen, alkyl, aryl,alkoxy-, --NR₄ --COR₅, --(CH₂)_(m) --NR₈ R₉, --(CH₂)_(n) --CONR₁₃ R₁₄ or##STR30## or a bridge member or a direct bond to a polymer chain, R₄,R₆, R₇, R₉, R₁₄, R₁₅, R₁₇, R₁₈ and R₁₉ being hydrogen or C₁ -C₄ alkyl,R₅ represents hydrogen, C₁ -C₄ alkyl or NR₆ R₇, R₈ represents --COR₁₀,R₁₀ represents NR₁₁ R₁₂, R₁₁ represents C₁ -C₄ alkyl or aryl, R₁₂represents hydrogen, C₁ -C₄ alkyl or aryl, R₁₃ represents hydrogen, C₁-C₄ alkyl or aryl, R₁₆ represents hydrogen, C₁ -C₄ alkyl, COR₁₈ orCONHR₁₉, m is a number of from 1 to 3, n is a number of from 0 to 3, pis a number of from 2 to 3 and Y represents O or NR₁₇ or R₁₃ and R₁₄together represent the atoms required to complete an optionallysubstituted heterocyclic ring, Z represents the C atoms required tocomplete a 5- or 6-membered aromatic heterocyclic ring, optionally witha fused benzene ring, and X.sup.⊖ is an anion which drops out where ananionic group is already attached to the rest of the molecule.